[EMSUSD-3040] Maya Autosaved file no longer has USD dirty layers contents

**Describe the bug**

Hi,

We found that since v0.31.0 ([PR-3982](https://github.com/Autodesk/maya-usd/pull/3982)), Maya autosaved file no long has USD dirty layers content; Manually save the scene file looks good though (assuming https://github.com/Autodesk/maya-usd/issues/4364 is fixed).

When Maya autosave is running, [`MSceneMessage::kBeforeExportCheck`](https://github.com/Autodesk/maya-usd/pull/3982/files#diff-7f61f7dfffdcfe9e57608606ddc989c5d0f5ffb2cb0886750d1c09f156cd07e5R340) is emitted but the [`LayerDatabase::prepareForExportCheck()` callback ](https://github.com/Autodesk/maya-usd/pull/3982/files#diff-7f61f7dfffdcfe9e57608606ddc989c5d0f5ffb2cb0886750d1c09f156cd07e5R458) no longer saving USD dirty layers.

**Steps to reproduce**
Steps to reproduce the behavior:
1. Launch Maya and open any USD file
2. Set edit target layer to be session layer
3. Make some changes to dirty the session layer
4. Manually click the 'Save' to save the scene file, set the option "Save all edits (including USD) to the Maya scene file"
5. Shorten the Maya autosave time (or wait for the autosave timer timeout)
6. Reopen the autosaved Maya scene file
7. The session layer content would be gone

**Expected behavior**
Expect the autosave file to have USD dirty layers content, the same as a manual save scene file.

**Specs (if applicable):**
- OS & version: Rocky Linux 9.4
- Compiler & version : gcc-11.4
- Maya version: Maya 2024.2
- Maya USD : official v0.31.0 / v0.33.0
- Pixar USD : official v24.03 / v25.08